In physically large electrical machines it is quite common to use two-layer coils (also known as “diamond” or “pulled diamond” coils). However, in physically large low-speed electrical machines with a low number of slots-per-pole-per-phase it can be cheaper to use single-layer coils (also known as “concentric” coils). An example of a fully formed conventional single-layer coil for use with open slots is shown in FIG. 1. The coil is formed from one or more insulated conductors as a complete loop and has several turns. The coil is received in slots provided in a surface of a stator and includes axially-extending winding runs 2 that are received within the slots. The winding runs 2 are joined together at each end of the coil by endwindings 4 that protrude out of the ends of the stator.
Conventional two-tier and three-tier stator windings are shown in FIG. 2. In these stator windings the endwindings of the coils forming the inner tier or tiers (i.e. the tier or tiers closest to the end of the stator) are bent away from the axis of the stator by up to 90 degrees so that they may pass over the coils forming the outer tier or tiers. The endwindings of the coils forming the outermost tier do not need to pass over other coils and can be substantially parallel to the axis of the stator or, as shown in FIG. 2, they can be bent away from the axis of the stator by up to 90 degrees.
It is important to note that the orientation of both endwindings of each individual coil with respect to the axis of the electrical machine are always the same. Thus, in the case of the coils forming the outermost tier of a two-tier or three-tier stator winding then both endwindings of each individual coil will either be substantially parallel to the axis of the stator or bent away from the axis of the stator by up to 90 degrees. In the case of the coils forming the inner tier or tiers then both endwindings of each individual coil will be bent away from the axis of the stator by up to 90 degrees so that they may pass over the coils forming the outer tier or tiers.
The coils can be inserted in slots 6 provided in the radially inner surface of the stator 8 and can be arranged concentrically in groups as shown in FIG. 2.